1. FIELD OF THE INVENTION
This invention relates to a lifting device for the closure plate of compressor valves for regulating the quanitity conveyed, with a gripper which is movably guided in the lifting direction and is acted upon in the opening direction of the closure plate by a regulating spring with tension variable by a setting device and which acts upon the closure plate by way of lifting prongs.
2. THE PRIOR ART
The object of the lifting devices used in reciprocating compressors for regulating the quantity conveyed is to delay the closure of the suction valves so that part of the medium drawn in is forced back by the piston into the suction duct. In this connection the lifting gripper participates in the lifting movements of the closure plate. During the opening of the valve the gripper strikes the lifting stop, e.g. on the catcher, whereas during the closing movement, as soon as the closure plate strikes the valve seat, it can disengage itself therefrom and swing out for the most part freely. In this connection the mass of the gripper should not exceed a certain upper limit dependent upon the structural features of the valve, so that during the shock of opening there may be no damage to or destruction of the valve. The gripper mass, however, should not be too small, as otherwise the speed of the gripper becomes too great during the closure procedure, in which the flow forces act upon the closure plate in an accelerating manner, and this in turn results in considerable stresses, in particular a hard impact of the closure plate upon the valve seat. In difficult operating conditions it can happen that the two mass limits are only a slight distance away from one another or even overlap one another, which can lead to overstressing of the lifting device and the suction valve.
German Patent No. 923 082 discloses a lifting device with a rigid gripper on which are secured sleeves in which pins acting as lifting prongs are guided. Shock-absorbing intermediate members are provided between the said pins and the sleeves, for example cylindrical pieces of soft rubber or plastics material can be inserted, stuck or vulcanized into the sleeves. The pins can be displaced independently of one another under the action of the shocks acting upon them during the regulating procedure. The stresses upon the suction valve and the lifting device itself cannot, however, be reduced in this way. It is particularly disadvantageous, however, for the closure plate not to be guided precisely during the regulating movement, so that it can be positioned obliquely under the action of the flow forces and performs a wobbly lifting movement.